A highly purified DNA ligase from rat liver nuclei has been tested on DNA in several types of complexes chosen to serve as models for chromatin, including complexes of polylysine or histones with DNA as well as reconstituted chromatin preparations. The limit of ligase sealing was measured as a function of the ratio of polypeptide or protein to DNA. With an excess of either polylysine or histones, the ligase is totally prevented from sealing nicks in the DNA. However, at ratios of histones to DNA similar to those occurring in chromatin, about half of the nicks are accessible to the ligase. When the chromatin is reconstituted at protein to DNA ratios similar to those occurring in chromatin, once more only about half of the nicks are accessible to the ligase. The rate of ligase action has also been measured on a variety of the complexes. When reconstituted chromatin is treated with an excess of DNA ligase, about half of the nicks are sealed. If this partially sealed preparation is taken through a second cycle of reconstitution, a large fraction of the previously unsealed nicks are now sealed upon exposure to the DNA ligase. The changes in the fraction of accessible nicks are in quantitative agreement with a random binding model in which all DNA sequences are equally suitable for binding of histones and rule out the binding of histones to a specific subset of DNA sequences in these model chromatin preparations.